The present invention relates generally to ceramic superconductors, and more particularly to a process for easily making a superconducting matrix of YBa.sub.2 Cu.sub.3 O.sub.7-x with fine and homogeneously dispersed Y.sub.2 BaCuO.sub.5 inclusions.
As described in U.S. Pat. No. 5,084,436 to Morimoto et al., which is incorporated by reference, a rare earth oxide superconductor such as YBa.sub.2 Cu.sub.3 O.sub.7-x (also referred to as a 1-2-3 phase; both nomenclatures are used interchangeably in this description) has a predetermined direction in the crystals of the 1-2-3 phase in which an electric current readily flows. The current tends to hardly flow among crystals aligned in different directions. Further, the grain boundaries can act either as poor superconductors or as insulating layers. Because of this, none of the polycrystal rare earth superconductors exhibit a high critical current density.
As described in Morimoto et al., to obtain a material having a high critical current density, the prior art has attempted a variety of methods for preparing a crystal structure where crystals of the 1-2-3 phase are aligned and grain boundaries are suppressed. The conventional method has been to solidify a melt of ceramic material in one direction under a temperature gradient to obtain an aligned ceramic material having a density higher than that obtainable by a sintering method. Unfortunately, the 1-2-3 phase melts incongruently at its peritectic temperature (the temperature at which part of the material is in a solid phase and part in a liquid phase) of about 1015.degree. C. to form Y.sub.2 BaCuO.sub.5 crystals (also referred to as a 2-1-1 phase; both nomenclatures are used interchangeably in this description) and a liquid phase rich in barium and copper. Accordingly, when this melt is cooled, a dense 1-2-3 material is formed with 2-1-1 inclusions. These inclusions are large in size and inhomogeneously distributed, which prevents good current density.
Morimoto et al. reported that although the 2-1-1 crystal particle inclusions show no superconductivity, they do not hinder the flow of electric current and give no substantial adverse effects to the superconducting properties as long as they are independent from each other. It has been since discovered that fine, homogeneously distributed, 2-1-1 inclusions actually help the superconducting material get good flux-pinning which results in higher current densities.
The present state of the art for producing 1-2-3 material with fine 2-1-1 inclusions involves mixing 1-2-3 powders with 2-1-1 powders or platinum. The powders are heated to &gt;1200.degree. C. and splat-quenched. Splat-quenching is cooling by quickly squeezing the material between two copper plates. After splat-quenching, the material is ground into powders and pressed into pellets. The pellets then undergo melt-processing where they are heated to a temperature where a melt or liquid begins to form. This temperature will generally be slightly above the peritectic point. This temperature is held for a short period (about 12 minutes), after which the pellets are quickly cooled to just below the peritectic point and further cooled at a slow rate. After slow-cooling to about 925.degree. C., the material is cooled to room temperature at about 240.degree. /hour. Finally, the pellets undergo an oxygenation process for 24 hours at 450.degree. C. Unfortunately, the whole process is very laborious, typically taking 4-5 days to complete.
Thus it is seen that there is a need for an easier method for making 1-2-3 material with fine and homogeneously dispersed 2-1-1 inclusions.
It is, therefore, a principal object of the present invention to provide a method for easily making a superconducting matrix of YBa.sub.2 Cu.sub.3 O.sub.7-x (1-2-3) with fine and homogeneously dispersed Y.sub.2 BaCuO.sub.5 (2-1-1) inclusions. The fine and homogeneously dispersed 2-1-1 inclusions provide increased flux pinning for higher current densities.
It is a feature of the present invention that it avoids the splat-quench step of the prior art melt-processing procedure.
It is an advantage of the present invention that it can produce finely distributed 2-1-1 inclusions smaller than one micron.
It is another advantage of the present invention that it can be used to easily produce very large pieces of superconducting material, such as bars, with fine and homogeneous distribution of 2-1-1 inclusions. It can also be used to easily produce superconducting magnets, superconducting leads such as downleads for large magnets, superconducting generators, actuators and bearings.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.